Vehicle fuel systems are required to control emission of fuel vapor. This is done by collecting vapor emitted from the fuel tank in a purge canister containing carbon to absorb the vapor. The canister is purged of collected vapor when the engine is running by drawing air through the canister into the engine, relying on manifold vacuum. The system is sealed except for venting to the atmosphere via the purge canister. On-board evaporative leak testing is required to ensure that leakage from the sealed system does not exceed acceptable limits. Typical known leak testing systems are described U.S. Pat. Nos. 5,333,590 and 5,765,121.
The latter patent describes a basic test in which the manifold vacuum is used to pump out the fuel tank and the return of tank pressure to atmospheric ("bleedup") is monitored. If bleedup exceeds a certain threshold value R the system is determined to have an unacceptable leak. If the bleedup is less than R, it assumed that there is no such leak. Leaks of less that a certain size cannot be reliably detected with this basic system because vapor generation from fuel in the tank can cause pressure in the evacuated system to recover more rapidly than small leaks. Thus, in order to improve the sensitivity of the basic bleedup test, measures must be taken to correct for different operating conditions, particularly the rate of vapor generation in the tank which causes a more or less gradual increase in pressure in a sealed tank, even where there is no leak, and may give a false indication of leakage.
For example, U.S. Pat. No. 5,333,590 uses a threshold value R which is not fixed but is related to vapor volume and fuel temperature.
The present invention seeks to make further improvements to evaporative fuel system leak testing to enable smaller leaks to be reliably detected under varying ambient and operating conditions.